swig and python3: surplus underscore

Question

Using swig 2.0.8 and python 3.2, running

swig -python -modern -py3 -o mymodule_wrap.c mymodule.i

produces a wrapper file that has

#  define SWIG_init    PyInit__mymodule

in there (note the two underscores between PyInit and mymodule).

Importing fails with

python3 -c "import mymodule"
Traceback (most recent call last):
   File "<string>", line 1, in <module>
ImportError: dynamic module does not define init function (PyInit_mymodule)

(note the single underscore).

Manually deleting the underscore in mymodule_wrap.c and recompiling results in a working module.

In this question: SWIG and Python3 Import Error the python interpreter complained about not finding PyInit__module.

What's wrong?

Answer 1

Make sure the extension module is named _mymodule.pyd not mymodule.pyd.

Explanation:

Given a SWIG .i file containing the declaration:

%module mymodule

SWIG will generate two files:

• mymodule.py
• mymodule_wrap.c

mymodule.py is imported into Python via import mymodule and loads _mymodule.pyd.

mymodule_wrap.c contains an entry point function PyInit__mymodule. This source file must be linked into the final _mymodule.pyd.

Python's import <module> statement looks for:

1. <module>.pyd with entry point PyInit_<module>.
2. <module>.py.

For a SWIG-generated extension, import mymodule will load mymodule.py, which loads _mymodule.pyd and looks correctly for PyInit__mymodule.

If the wrong extension name is used, import mymodule will load mymodule.pyd and look incorrectly for PyInit_mymodule.